This invention relates to a process for separating alkyl carbonate from feedstocks containing alkyl carbonate and water. More particularly, this invention relates to a liquid-liquid extraction process for separating alkyl carbonate from feedstocks comprising alkyl carbonate, water, and impurities such as unconverted alkanol from an alkyl carbonate production process for uses such as the production of high oxygen-content gasoline blending components (Oxygenates).
Oxygenates have been part of the United States gasoline strategy since the late 1970s. With the Clean Air Act Amendments of 1990, the demand for oxygenates is expected to increase even further. For example, starting in the winter months of 1992, gasoline containing 2.7 weight percent oxygen will have to be provided to approximately 40 metropolitan areas that have failed to meet carbon monoxide pollution standards. It is expected that in the near future, between 30 and 60 percent of the United States gasoline pool may be required to contain oxygenates.
The most commonly used oxygenates today are methanol, ethanol, and methyl tertiary butyl ether (MTBE). Although methanol and ethanol have high blending octanes, problems with toxicity, water miscibility, high Reid Vapor Pressure (RVP), high nitrogen oxide emissions, lower fuel efficiency, and cost have dampened industry enthusiasm for these components. Partially as a result of the above, MTBE has become particularly attractive.
Homologues of MTBE such as ethyl tertiary butyl ether (ETBE) and methyl tertiary amyl ether (TAME) are also gaining industry acceptance. Moreover, commercial activity with respect to ETBE and TAME is expected to increase relative to MTBE, in view of the recent Environmental Protection Agency decision to reduce the RVP requirements for gasolines well below 9 psia, the blending RVP of MTBE.
Oxygenate production capacity, however, is limited by ether plant capacity and by feedstock availability. MTBE and ETBE both utilize isobutylene as a feedstock while TAME uses isoamylene as a feedstock. Isobutylene and isoamylene are generally supplied to an ether facility in a petroleum refinery from a fluid catalytic cracking unit (FCC), a fluidized or delayed coker, or from downstream paraffin isomerization and dehydrogenation facilities. The availability of hydrocarbons having 4 carbons is limited by constraints such as, but not limited to, crude properties, FCC catalyst properties, FCC operating conditions, coking conditions, as well as other refinery operating constraints. The chemical mix of C.sub.4 and C.sub.5 paraffins, olefins, and aromatics as well as the particular mix of iso-olefins to normal olefins are similarly constrained.
Thus, there exists a great need in the petroleum industry for a low-cost method of increasing oxygenate production capacity that overcomes or avoids the obstacles described above.
The use of carbonates, and particularly the dialkyl carbonates in fuels has been the subject of several patents and patent applications.
European Patent Application Numbers 0 082 688 to Bretherick and 0 098 691 to Spencer disclose the use of dialkyl carbonate and dimethyl carbonate in fuels for use with spark ignition engines.
U.S. Pat. No. 4,380,455 to Smith discloses the use of dialkyl carbonates for preventing the phase separation of hydrous ethanol from liquid hydrocarbon fuel mixtures.
U.S. Pat. No. 4,891,049 to Dillon discloses the use of non-aromatic metals-free carbonates for reducing particulate emissions from distillate-based fuels such as diesel fuel and jet fuel.
Carbonates can be produced using any of several methods known in the art, each method having attendant advantages and penalties. Such methods include the carbonylation of alcohols, alkylene carbonate alcoholysis, urea alcoholysis, inorganic methods, and phosgene alcoholysis.
One of the oldest methods for manufacturing carbonates employs phosgene. The phosgene is generally contacted with methanol to form methyl chloroformate in accordance with the following reaction: EQU CH.sub.3 OH+COCl.sub.2 .fwdarw.CH.sub.3 OCOCl+HCl
The methyl chloroformate reacts with an additional mole of methanol to form dimethyl carbonate as follows: EQU CH.sub.3 OCOCl+CH.sub.3 OH.fwdarw.CH.sub.3 OCOOCH.sub.3 +HCl
One associated penalty with the process described hereabove is that the process requires the use of toxic phosgene. Moreover, the method also leads to the coproduction of other chloride-containing by-products such as alkyl chlorides, which can often be toxic themselves. Chlorine-containing by-products such as hydrogen chloride can also be particularly corrosive. Neutralization methods to balance the acidity of such chlorided components in order to mitigate such corrosive effects such as the addition of sodium hydroxide to drive the reaction by the production of sodium chloride and water, can be costly and can also compromise product quality.
Oxidative carbonylation is another method that has been used to produce carbonates. It is generally known that carbonates can be produced from alkanol and carbon monoxide in the presence of certain metal chlorides or metal alkoxy chlorides through an oxidation-reduction reaction. An example of such a reaction with methanol and carbon monoxide over a copper chloride catalyst is as follows:
Oxidation: EQU 2CH.sub.3 OH+1/2O.sub.2 +2CuCl.fwdarw.2Cu(OCH.sub.3)(Cl)+H.sub.2 O
Reduction: EQU 2Cu(OCH.sub.3)(Cl)+CO.fwdarw.2CuCl+CH.sub.3 O(CO)OCH.sub.3
Overall: ##STR1##
U.S. Pat. No. 4,218,391 to Romano et al. discloses such a process for the production of carbonates comprising reacting an alkanol with oxygen and carbon monoxide in the presence of a catalyst consisting of a copper metal salt of the group of cuprous and cupric salts having a single inorganic anion.
U.S. Pat. No. 5,004,827 to Curnutt discloses a similar process for the production of carbonates comprising contacting an alkanol with carbon monoxide and oxygen in the presence of a heterogeneous catalyst comprising a metal halide such as cupric chloride with or without potassium chloride impregnated on an appropriate support such as activated carbon.
This reaction cannot generally be operated to high conversion because high concentrations of water in the reactor lead to low selectivity, i.e., high CO.sub.2 yields. Additionally, excess water can lead to the formation of a variety of copper hydroxy chloride phases of the formula Cu(Cl).sub.x (OH).sub.y.nH.sub.2 O. None of these phases are particularly effective for the production of carbonates.
For these, among other reasons, it is generally desirable that these reactions be conducted with a low conversion for each pass through the reactor with an effective strategy for feed/product separation and recycle of the unconverted feed.
Moreover, under typical oxidative carbonylation reaction conditions, the product stream generally comprises the alkyl carbonate, alkanol, and water, wherein the alkyl carbonate can form azeotropes with both the alkanol and water. For example, Table 1 illustrates the boiling point and composition of various binary azeotropes within a mixture comprising methanol, ethanol, dimethyl or diethyl carbonate, and water.
TABLE 1 ______________________________________ Azeotrope Comp.- Boil. Pt. Temp. .degree.C. Pure Comp. Wt % (@ 14.7 psia) ______________________________________ 62.7 70% MeOH + 30% DMC 65.0 MeOH 74.0 55% DMC + 45% EtOH 77.5 89% DMC + 11% H.sub.2 O 78.2 96% EtOH + 4% H.sub.2 O 78.3 EtOH 90.0 DMC 91.0 70% DEC + 30% H.sub.2 O 100.0 H.sub.2 O 126.0 DEC ______________________________________
The formation of these various azeotropes complicates downstream separation steps such that they cannot be easily or effectively performed by conventional distillation methods.
Several methods have been suggested to overcome these product separation problems.
U.S. Pat. No. 3,963,586 to Ginnasi et al. discloses a process for separating dimethyl carbonate from a mixture of dimethyl carbonate, methyl alcohol, and water. In the disclosed process, water is fed to the top of an extraction distillation column at a water solvent to feed ratio by weight of at least 10:1. A stream of methyl alcohol and water with minor amounts of dimethyl carbonate is withdrawn from the bottom of the column while an overhead stream containing dimethyl carbonate, water, and a minor amount of methyl alcohol is directed overhead. The overhead product stream is generally cooled and decanted into a bottom organic phase containing dimethyl carbonate (97%) and water (3%) with a minor amount of methyl alcohol and a top aqueous phase containing a substantial amount of water (87%) with dimethyl carbonate (12%) and methyl alcohol (1%).
Such processes require extensive retrofitting and modifications in order to produce fuel blending components. For example, the organic phase containing 97% dimethyl carbonate and 3% water still generally requires desiccation or reprocessing in a water fractionation step or through other means to reduce the water content of the fuel sufficiently for fuels blending. Similarly, the aqueous phase containing 12% dimethyl carbonate must be undesirably reprocessed to the extraction distillation column along with the water component, at the expense of energy and capacity penalties. Moreover, processes that operate at a solvent to feed ratio by weight in excess of 10:1 can require distillation extraction columns having excessively large and thereby costly vessel diameters.
U.S. Pat. No. 4,162,200 to Himmele et al. discloses a process for obtaining dimethyl carbonate from its solutions in methanol by extractive distillation with an aprotic extractant at a column temperature profile ranging from 60.degree. F. at the top of the column to 250.degree. F. at the bottom. The methanol is generally carried upwards to the overhead product stream, leaving a bottoms product containing the dimethyl carbonate and aprotic extractant. The aprotic extractant is further characterized as:
(a) being substantially inert towards dimethyl carbonate, PA1 (b) boiling at a temperature above 100.degree. C. at atmospheric pressure, PA1 (c) being miscible with dimethyl carbonate in all proportions, PA1 (d) having a dielectric constant, .epsilon., of from 4 to 90, and PA1 (e) having a dipole moment, .mu., of from 1.5 to 5 Debye.
The aprotic extractant is also added at the high extractant to dimethyl carbonate weight ratio of 0.5 to 50 kilograms of extractant per kilogram of dimethyl carbonate. The large volumes of aprotic extractants generally must be separated from the products and recovered at substantial expense to the refiner since these extractants can be costly and furthermore, can contaminate the finished product.
Moreover, processes requiring extractive distillation for alkyl carbonate separation such as those described above, generally require additional processing equipment such as reboilers, overhead condensers, overhead reflux drums, and phase separators in order to be effective. These systems are particularly costly to procure and erect. Extractive distillation towers for alkyl carbonate separation also require that an appropriate temperature profile be maintained across the column in order to obtain the desired distillation product cutpoints. These distillation temperature profiles generally conflict with maintaining optimum extraction process temperatures, resulting in cost penalties.
U.S. Pat. No. 4,428,753 to Bezman discloses a process for separating a binary mixture of water and isopropanol. The process requires mixing the aqueous isopropanol feedstock with gasoline blending hydrocarbons and coalescing the mixture in a mechanical separation coalescer. Such aqueous isopropanol separation processes are particularly difficult to operate since the gasoline blending hydrocarbon/isopropanol mixture is prone to emulsion formation. Breaking the emulsion and effectively separating the gasoline blending hydrocarbon/isopropanol and water mixture cannot generally be achieved without exotic and costly separation devices such as separating membranes, electrical devices, and/or beds of granular particulate.
Methods for concentrating solutions containing an alcohol and oxygenates such as organic ethers, aldehydes, ketones, and esters using physical separation means are also known in the art.
U.S. Pat. No. 4,798,674 to Pasternak et al. discloses a process for concentrating mixtures containing dimethyl carbonate and methanol using a membrane-based pervaporation step.
"Opportunities For Membranes in the Production of Octane Enhancers," by Shah et al., AICHE Spring 1989 National Meeting, Symposium Series, Vol. 85, No. 272, pgs. 93-97, also discloses a process for separating dimethyl carbonate and methanol azeotropes across a membrane using pervaporation.
However, physical membrane separation systems such as those described have not been effectively used in binary systems wherein water and alkyl carbonate or water and alkanol have been the constituents. Moreover, membrane systems have not been particularly reliable in industrial environments and have been known to present operability problems.
It has now been found that feedstocks comprising alkyl carbonate and water can be separated using liquid-liquid extraction, into an extract stream comprising a substantial amount of the alkyl carbonate present in the feedstock and a raffinate stream comprising a substantial amount of the water present in the feedstock. The liquid-liquid process is conducted with a hydrocarbon extraction solvent selective for alkyl carbonate relative to water.
It has also been found that a hydrocarbon extraction solvent selective for extracting alkyl carbonate over alkanol having a specific gravity of greater than 0.60 and less than 1.0 provides enhanced liquid-liquid extraction performance through improved raffinate/extract disengaging.
It has also been found that a solvent selective for extracting alkyl carbonates having an aromatic hydrocarbon concentration by weight of greater than 1 percent and an olefinic hydrocarbon concentration by weight of below 80 percent, provides improved extraction of alkyl carbonate from a feedstock comprising alkyl carbonate and water.
It has also been found that several hydrocarbon streams generally available in conventional petroleum refineries are suitable hydrocarbon extraction solvents including, but not limited to catalytic reformate, catalytic cracking process naphtha, heavy virgin naphtha, isomerate, alkylate, and the aromatic derivatives of various refinery streams including benzene, toluene, xylene, and combinations thereof.
It is therefore an object of the present invention to provide an extraction process for a feedstock comprising alkyl carbonate and water that recovers a substantial portion of the alkyl carbonate to an extract product stream and a substantial portion of the water product to a raffinate product stream.
It is another object of the present invention to provide a process that does not require costly extractive distillation columns for alkyl carbonate separation or require distillation column temperature profiles that are inconsistent with optimum extraction temperatures.
It is yet another object of the present invention to provide a process that does not require physical separation means, such as membrane systems, which can be unreliable in an industrial environment.
It is still another object of the present invention to provide a process that does not require the use of large volumes of costly aprotic polar solvents, utilizes several hydrocarbon streams generally present in a petroleum refinery, and does not require complete recovery of the hydrocarbon extraction solvent where the alkyl carbonate is to be blended for fuels use.
Other objects appear herein.